Polyisocyanate mixtures containing diphenylmethane diisocyanates (MDI) and higher functional polymethylene polyphenylpolyisocyanates (polymeric or crude MDI) are known. For example, the general synthesis product of the phosgenation of polyphenyl amines yields, depending upon conditions such as catalyst type, temperature and the like, a mixture of oligomers and isomers. U.S. Pat. No. 3,344,162 describes the production of aromatic polyisocyanates containing from about 50 to about 95 percent by weight of the diisocyanate of which from about 10 percent to about 95 percent by weight is the 2,4'-isomer. U.S. Pat. No. 3,362,979 claims a mixture of polyisocyanates containing from 20 percent to 100 percent of the diisocyanates of which 20 percent to 95 percent by weight is the 2,4'-isomer. U.S. Pat. No. 3,960,788 describes a mixture of polymethylene polyphenylpolyisocyanates having an average functionality of about 2.1 to about 2.4 of from about 60 percent to 75 percent by weight MDI and from 25 percent to about 40 percent higher functional polymethylene polyphenylisocyanates. The diphenylmethane diisocyanate has an isomer content of about 60 percent to about 80 percent 4,4'-isomer, about 18 percent to about 33 percent 2,4'-isomer, and about 2 percent to 7 percent 2,2'-isomer. These synthesis mixtures are disclosed as having a viscosity of about 60 cps at 25.degree. C. U.S. Pat. No. 4,256,849 claims a process for the production of cold setting flexible foams wherein a mixture of diphenylmethane diisocyanates and polyphenylpolymethylene polyisocyanates containing from 60 to 90 percent by weight of 4,4'-diphenylmethane diisocyanate and from 3 to 30 percent by weight of 2,4'-diphenylmethane diisocyanate is used. It further discloses the amount of polyphenylpolymethylene polyisocyanates to be from 0 to 37 percent by weight. Unfortunately, the synthesis mixtures having a high content (50 percent to 100 percent by weight) of the 2-ring MDI isomer, where the major isomer is 4,4'-, tend to form insoluble crystal dimers in solution. The product, in turn, must then be filtered to remove the unreactive solids.